The Protocab Pilot Series is ideal for 4mm scales (e.g. P4, 00, H0), and the components will fit happily in a 4mm steam locomotive's tender, and will even fit (at a bit of a squeeze it has to be said!) in some larger 4mm tank engines.
To show this, you may find some of the Customer Installation Reports of interest, kindly written for us by members of Club Protocab many of whom who have installed Protocab in their 4mm locos.
The larger of the two batteries available in the Pilot Series will fit (usually at a bit of an angle) in a 4mm steam loco tender. For tank engines, probably the smaller battery would fit the bill better. We always say, if you can fit the larger battery, that's the best option as you get larger capacity and more power, however the smaller battery is more than functional and itself gives a couple of hours of running on a full charge.